This invention concerns sewage treatment, and relates particularly to an enhancement of a known process, eliminating a step normally included on the liquid side of a wastewater treatment plant, with attendant savings in space and cost.
On the liquid side of a conventional activated sludge sewage treatment plant, nitrification and denitrification often occur in several tanks that successively handle the influent wastewater, with recycling of sludge from an aerobic tank to an anoxic tank. A portion of the wastewater exiting the aerobic tank is not recycled to the anoxic stage and normally is subjected to secondary clarification, in one or more clarifier tanks.
In one prior art process, the secondary clarification was eliminated by use of membrane separators. An activated sludge process incorporating membranes for the purpose of filtration to eliminate the need for subsequent clarification is referred to as a membrane bioreactor (MBR). This known technology is described, for example, in U.S. Pat. No. 5,192,456. An MBR plant designed for removing nitrogen biologically usually includes a separate anoxic stage followed by an aerobic stage, which is the MBR. This process requires that mixed liquor be recycled from the aerobic stage back to the anoxic stage to maintain bacterial populations, and to provide nitrates for the anoxic stage. In this way, in the anoxic stage, nitrogen gas is released as microbes obtain oxygen from nitrates coming from the aerobic stage via the recycle. Nitrification occurs in the aerobic (MBR) stage.
Such a prior MBR process is schematically indicated in FIG. 1 of the drawings.
Another, different activated sludge process, known as the SymBio process, uses measurement and adjustment of process parameters to maintain low oxygen concentration below 1.0 mg/1 allowing simultaneous nitrification and denitrification in a single treatment tank and thus eliminating need for a separate anoxic stage, as well as the need for mixed liquor recycle. Further, maintaining low oxygen concentration below 1.0 mg/1 reduces the energy requirements for the desired treatment. However, the SymBio process has typically been used in conjunction with a separate clarification step to isolate solids from the treated water, and the separated solids are recycled back to the treatment tank to maintain bacterial population in the tank. The SymBio process is described in U.S. Pat. No. 5,906,746, which is incorporated herein by reference.
In the treatment process and system of this invention, the MBR process described above is combined with the SymBio technology of U.S. Pat. No. 5,906,746, to eliminate the need for both recycle and clarification and to minimize tank requirements. The process and the system of the invention can significantly reduce the footprint, energy consumption, sludge production, chemical consumption and capital cost of a wastewater treatment plant.
Pursuant to the present invention, the advantages of both the SymBio process and the MBR reactor process are realized, with nitrification and denitrification achieved in a single tank and with membrane separators in the same tank so as to eliminate the need for separate clarification. The process of the invention goes beyond both the prior processes mentioned by enabling the nitrification/denitrification process, and the removal of water, to be accomplished in a single tank if desired. Still further, the new process makes an additional improvement over an MBR reactor system in that additional process air, often needed in MBR systems to satisfy the oxygen demand of the biomass to oxidize the organic contaminants, is significantly reduced as it is regulated by the SymBio process. In a typical MBR reactor, air is provided for two purposes: (a) to keep the membranes clean by agitation with the air; and (b) to satisfy oxygen demand of the microorganisms and to remove the organic contaminants. The MBR system needs a fixed amount of air to keep the membranes clean. This is dictated by the quantity or square footage of the membrane separators. Normally, however, this quantity of air is not sufficient to meet the needs of the microorganisms to remove the contaminants, and additional process air must be introduced. The supplemental process air can be supplied within the MBR tank, or it is often supplied in a separate aerobic tank. By combining the SymBio process with the MBR technology, the invention reduces and minimizes process oxygen requirements and hence reduces the supplemental air requirements. It is noted that with the invention the membranes need not necessarily be cleaned by air; other means such as liquid recirculation can also be used.
As noted above, the need for recycle of sludge is eliminated by the invention, because the SymBio process combines nitrification and denitrification within a single tank. Separate clarification is also eliminated, and along with it the recycle of sludge from a clarifier back to the anoxic tank, or the anoxic/aerobic tank, is also eliminated. The invention can in this way make significant reduction of the footprint, energy consumption, capital cost and chemical consumption of a wastewater treatment plant. These and other objects, advantages and features of the invention will be apparent from the following description of a preferred embodiment, considered with the drawings.